1. Technical Field
This invention relates to a resonator element for exciting a thickness-shear vibration, a resonator, an electronic device, an electronic apparatus, and a mobile object.
2. Related Art
An AT-cut quartz crystal resonator for exciting a thickness-shear vibration as a vibration mode of the principal vibration is suitable for miniaturization and higher frequency, and has a frequency-temperature characteristic showing an excellent cubic curve, and is therefore used in a variety of fields such as an oscillator and electronic equipment. In particular, in recent years, due to the progress in speeding up of the processing speed of transmission communication equipment and OA equipment, or increase in capacity of communication data and a processing amount, a demand of increase in frequency to the AT-cut quartz crystal resonator as a reference frequency signal source used therefor is increasing. In general, in order to realize the AT-cut quartz crystal resonator with a higher frequency excited in the thickness-shear vibration mode, the thickness of the vibrating portion is decreased to thereby achieve the higher frequency.
However, if the thickness of the vibrating portion is decreased for achieving the higher frequency, there arises a problem that the adjustment sensitivity of the frequency increases to degrade the frequency tuning accuracy, and thus the production yield of the resonator is degraded. To cope with this problem, JP-A-2002-111435 (Document 1) discloses the fact that the margin in tuning the frequency can be increased in the resonator of the temperature compensated oscillator by increasing the pulling sensitivity by decreasing the capacitance ratio γ (=C0/C1; here, C0 denotes an equivalent parallel capacitance, C1 denotes an equivalent series capacitance) of the resonator by roughly evenly cutting out the four corners of a rectangular excitation electrode so that the area ratio between before and after cutting out the excitation electrode is 95% through 98%.
Further, in the paragraph [0024] of Document 1, there is a description reading that “if the area ratio is lower than 95%, it results that the vibrating portion making an actual contribution to the vibration is cut out from the excitation electrode section, and the equivalent series capacitance C1 is decreased, which makes it unachievable to obtain the effect of decreasing the capacitance ratio γ.”
However, if the capacitance ratio γ can be decreased by cutting out the four corners of the excitation electrode to achieve the area ratio of 95% through 98% as disclosed in Document 1 mentioned above, although it is sufficient for the electrical characteristics of the temperature compensated oscillator, in the resonator used for the voltage controlled oscillator, there is a problem that it is unachievable to obtain the pulling sensitivity sufficient to compensate the frequency adjustment accuracy thus deteriorated.
Therefore, it is necessary to realize the resonator making it possible to enlarge the pull range of the voltage controlled oscillator in, for example, the high-frequency band (equal to or higher than 200 MHz).